In the continuation of this document, effort will be made more particularly to describe an existing set of problems in the field of digital terrestrial television broadcasting networks. The disclosure is of course not limited to this particular field of applications, but is of interest in any data transmission or broadcasting technique having to confront a close or similar set of problems, and particularly in broadcasting networks implementing the DVB-T or DVB-H standard.
Digital terrestrial television networks, also called DTT, which implement the DVB-T or DVB-H standard, are today deployed in France, in Europe and in other countries of the world. For the most part, these networks are of the MFN type (Multi-Frequency Network), which means that the various transmitters of such a network operate at separate frequencies. Conversely, in some geographic regions, the networks are of the SFN or isochronous type, which means that the various transmitters must be synchronised precisely in terms of time, frequency and content.
As a matter of fact, the operating principle of such SFN networks can consist in transmitting a single signal from at least two separate geographic sites at each of which a transmitter is located. The sought-after objective is then to add the contribution of these two signals in reception, which requires them to be received at the same instant, within a guard time based on the modulation profile and proportional to the symbol time width, and at the same frequency, in order to prevent them from interfering with one another. Based on the geographic distance of the receiver from each of the two transmitters, it is therefore sometimes necessary to take account of the different transport times of the signals, and more generally the propagation channel and the interferences that it is likely to introduce.
Due to this requirement for time and frequency synchronisation of the various transmitters, the implementation of such SFN networks proves to be particularly difficult.
Several methods have thus far been proposed for enabling DVB-T output signals from the transmitters of a DTT broadcasting network to be synchronised, which, for the most part, are based on time marking the data frames to transmit, as shown below in connection with FIG. 1. Such a marking method is standardised, and, for more information about this method, reference may be made to the standard referenced as ETSI TS 101 191.
In accordance with this standard, FIG. 1 shows a block diagram of a SFN-type digital terrestrial television broadcasting system implementing data broadcasting in the MPEG-2-TS format (Motion Picture Expert Group-Transport Stream).
Two transmitters 10 and 11 are shown in FIG. 1, which each include synchronisation equipment (SYNC system) 101, 111, and a DVB-T modulator 102, 112. The synchronisation equipment 101, 111 is supplied with two frequency and time reference signals, e.g., a signal corresponding to one pulse per second, or 1 pps, and a signal at 10 MHz resulting from the 1 pps. It is observed that there are exactly 10 million periods of the frequency reference signal at 10 MHz between two 1 pps pulses.
These signals can be derived from any reliable reference system 105 and 115, e.g., from the American GPS (Global Positioning System) or European Galileo positioning system, or from low-frequency radio frequency carriers (DCF77 in Germany, MSF in the United Kingdom, France Inter in France, etc., which are registered trademarks).
The 10 MHz reference frequency signal can likewise be used at the head end, by an SFN adapter, in order to calibrate the output flow thereof, so that it is stable and accurate, as well as by the broadcasting transmitting station transmitters, in order to calibrate the output thereof.
This reference can likewise be used by the transmitters in order to synchronise the transmitting frequency thereof, which must be accurate to within less than 1 Hz in DTT in an SFN network for optimal operation.
The data being broadcasted by each of these transmitters 10, 11 are received in the form of a transport stream of the MPEG-2 TS type (for MPEG-2 Transport Stream), derived from a receiver 12 likewise acting as a network adapter (RX network adapter).
Upstream, at the other end of the broadcast chain, the MPEG-2 TS stream of data to transmit is constructed by an MPEG-2 multiplexer referenced as 13, which creates the data frames. Such an MPEG-2 multiplexer, for example, is situated in a national head-end, from which the data to broadcast by each of the transmitters 10, 11 of the broadcasting network is next transported via satellite (in a transport network also called a distribution network). After MPEG-2 multiplexing 13, the data is processed by an SFN adapter 14, which time-marks the frames using the same time and frequency reference system 15 as the one 105, 115 used by the synchronisation equipment 101, 111 of the transmitters 10 and 11. In the transmission, the SFN adapter 14 is the pendant of the synchronisation equipment 101, 111 in reception. In this way, the SFN adapter is also supplied with a reference frequency signal at 10 MHz and with a reference time signal at one pulse per second.
Upon exiting the SFN adapter 14, the data stream is therefore of the MPEG-2 TS type: it is then transmitted by a network adapter 16 (TX network adapter), and conveyed by means of the transport or distribution network 17 (e.g., a satellite distribution network), to the receivers 12, so as to be made available to the transmitters 10 and 11.
More precisely, the time-marking carried out by the SFN adapter 14 consists, on the one hand, in constructing mega-frames, each corresponding to 8 DVB-T frames in 8K mode, or to 32 DVB-T frames in 2K mode, and, on the other hand, in inserting, at any location of each of these mega-frames, a mega-frame initialisation packet or MIP.
The MIP packet of the mega-frame of index n, referenced as MIPn, is identified by its own PID (Packet Identifier) and, in particular, includes:                a 2-byte word called a “pointer,” which provides the number of data packets (TS packets) between the current MIP and the first TS packet of the following mega-frame;        a 3-byte word called a “Synchronisation_time_stamp”, or STS, which provides the number of 10-MHz periods between the last reference 1 pps pulse preceding the beginning of the mega-frame of index n+1 and the beginning of this following mega-frame of index n+1 (identified by the first bit of the first packet of this mega-frame).        
FIG. 2 illustrates these various notions precisely for:                the output data stream of the head-end SFN adapter, referenced as 21;        the input data stream of the modulator on the transmission site, referenced as 22; and        the modulated signal broadcast by the transmitted, referenced as 23.        
As indicated previously, the SFN adapter 14 organises the data stream 21 into mega-frames, and inserts one and only one MIP packet per mega-frame (MIPn-1 for the mega-frame n−1, MIPn for the mega-frame n).
At the level of the transmitters 10, 11, the SYNC system module 101, 102 receives as input, on the one hand, the MPEG 22 stream which was transported in the network, and, on the other hand, the 1 pps and 10 MHz frequency reference originating, time reference example, from the GPS receiver.
It searches for the MIPn-1 packet.
Having found the MIPn-1 packet, and, owing to the “pointer” value, it finds the first TS packet of the following mega-frame n, referenced as TSn,1. Bit-level synchronisation was therefore carried out.
Having found the first packet TSn,1, the SYNC system module 101, 102, owing to the STS value and to the 1 pps pulse, determines at which moment this first TS packet of the following mega-frame exited the head-end SFN adapter. This corresponds to the transport delay (or time).
Finally, the SYNC system module 101, 102 deduces therefrom the moment of broadcasting, which corresponds to the moment of output from the head-end SFN adapter, to which a controlled delay common to all of the transmitters of the transmission sites is added (maximum delay or “Max_Delay,”, which is likewise transported in the MIP packets), as well as a delay which may be specific to each transmitter (“Tx_time_offset”).
In other words, the transmitters 10, 11 use MIP signalling and a time reference (e.g., a 1 pps signal), which is identical to that used at the head-end transmitter level, in order to carry out a comparative analysis of the MIP signalling and the STS time stamps, and to take the decision to more or less delay the frame received as output from the transmitter of the transmission site. In this way, this deterministic method, which is based on the same 1 pps time reference as at the head-end, ensures the time synchronisation of the signals output by the transmitters of the transmitting sites.
However, this synchronisation is only possible if, on the one hand, the transport time is less than the Max_Delay value (itself less than one second if a 1 pps time reference is used), and, on the other hand, if the 1 pps time references “pulse,” i.e., emit a pulse at the same moment in the various transmissions sites.
It is therefore necessary for the 1 pps time reference and 10 MHz frequency reference to be common at all points of the broadcast chain. They are therefore conventionally deduced from GPS data reception.
In order to reduce the cost of the equipment, attempts are currently being made to develop SFN networks which are not based on the use of GPS receivers at each transmission site.
For example, the document WO 2006/084361 proposes to insert time information into the data stream to broadcast, at the head-end level, to recover this information at each transmission site, and, from this time information, to generate a reference signal used by the various transmission sites in order to synchronise themselves.
However, this technique enables the various transmitters to be synchronised only if all of the transmissions sites use the same synchronisation technique. Consequently, this technique does not provide correct synchronisation of all the transmitters if some transmitters use a 1 pps reference generated using a GPS-type receiver, and other transmitters regenerate a reference signal from the time information carried in the broadcasted stream.
As a matter of fact, due to the variation in the position of the satellite used for transporting the data stream (MPEG-TS) from the head-end towards the various transmitting stations, the transport time between the head-end and the various transmission sites varies. For example, the transport time to a given transmitting station varies in time by approximately 270 μs, when considering a geostationary satellite situated at approximately 36,000 Km away, the position of which varying within a cube of approximately 80 Km per side.
The 1 pps time reference regenerated according to the technique of document WO 2006/084361 is therefore not synchronised with the 1 pps time reference derived from a GPS receiver. In other words, a deviation of Δ1 pps exists between the 1 pps time reference regenerated according to the technique of document WO 2006/084361 and the 1 pps time reference derived from a GPS receiver, with respect to two transmitters of a single SFN plate.
Furthermore, the movement of a satellite induces frequency variations. More precisely, due to the Doppler effect, the movement of the satellite and the speed thereof induce a jitter in the flow rate and therefore in the 10 MHz reference frequency. Consequently, the transmission frequencies of the various sites are liable to differ by a few hertz.
It therefore appears to be impossible to place within a single network, in an SFN, transmitters having time references which are not common at all points of the broadcast chain.